What is opportunity of SBook discrete video in compare to integrable one?

In Surface book there are both dedicated - NVIDIA Maxwell GPU Surface Book - and integrated - Iris 540 - videos. I've read, that these videos are comparable, at least - in gaming performance, so assuming that gaming performance is complete characteristic of video performance, I don't see the importance of dedicated GPU, and even don't understand why it is needed.

In Surface book there are both dedicated - NVIDIA Maxwell GPU Surface Book - and integrated - Iris 540 - videos. I've read, that these videos are comparable, at least - in gaming performance, so assuming that gaming performance is complete characteristic of video performance, I don't see the importance of dedicated GPU, and even don't understand why it is needed.